1. Field of the Invention
This invention relates generally to a fuel cell stack employing a low voltage tap and, more particularly, to a fuel cell stack having one or more low voltage power taps for providing low voltage DC power for low voltage devices in a fuel cell system or AC power to other devices in the system.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is disassociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The combination of the anode, cathode and membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Many fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for an automobile may have two hundred stacked fuel cells. The fuel cell stack receives a cathode input gas as a flow of air, typically forced through the stack by a compressor. Not all of the oxygen in the air is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several membranes in the stack. For the automotive fuel cell stack mentioned above, the stack would include about two hundred bipolar plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. The bipolar plates are made of a conductive material, such as stainless steel, so that they conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid and the anode and cathode gases for the electrochemical reaction flow, as is well understood in the art.
Vehicles and other systems typically require 12 volt DC nominal power to provide power to various vehicle accessories, such as headlights, switches, etc. In the known systems, the 12 volt DC power is provided by a DC/DC converter that down-converts the high voltage from the fuel cell stack. Particularly, the high voltage of the overall DC power from the fuel cell stack is down-converted by the DC/DC converter to the desired voltage level for the various low voltage devices. However, the DC/DC converter adds cost, mass, volume, losses and additional assembly costs to the fuel cell system. Further, in high voltage fuel cell systems where the fuel cell stack is floating with respect to the vehicle chassis ground, the DC/DC converter must include electrical isolation to prevent the chassis from coming in contact with the high voltage. This converter isolation also increases the cost, mass and losses associated with the system. It would be desirable to eliminate the DC/DC converter from the fuel cell system for these reasons.